Protein pI Study Guide
The isoelectric point describes one important state of a protein. At this pH, total positive charge equals total negative charge. The molecule has no net electrical charge. It may still contain charged sites. They simply balance each other. This balance affects solubility, binding, purification, and movement in a field.
Why Charged Groups Matter
Proteins carry charge through ionizable groups. The amino terminus can gain a proton. The carboxyl terminus can lose a proton. Side chains also matter. Aspartic acid, glutamic acid, cysteine, and tyrosine add acidic behavior. Histidine, lysine, and arginine add basic behavior. Each group has a pKa. The pKa tells when half of that group is charged.
How the Estimate Works
This calculator counts charged residues from the entered sequence. It then applies Henderson Hasselbalch style charge terms. Positive groups fade as pH rises. Negative groups grow as pH rises. The tool scans the selected range and uses bisection. It searches for the pH where net charge is near zero. This gives a practical pI estimate for study and planning.
Advanced Use Cases
Custom pKa values help when you compare sources. Different databases use different constants. Local structure can also shift pKa values. A buried residue may behave differently than a free amino acid. Salt, temperature, solvent, and nearby charges can change results. This page is best for quick theoretical screening. Experimental values may differ for real folded proteins.
Reading the Result
A high pI suggests more basic groups. A low pI suggests more acidic groups. The target pH charge helps predict behavior in buffers. If pH is below pI, the molecule tends to be positive. If pH is above pI, it tends to be negative. This idea helps in ion exchange planning and electrophoresis review.
Good Input Practice
Enter one letter amino acid codes only. Remove spaces, numbers, and labels. Include the mature sequence you want to test. Signal peptides, tags, and cleavage sites can change charge. Use the export buttons for lab notes, class reports, and comparison tables.
For repeated work, keep one pKa set consistent. This makes comparisons fair. Record blocked termini and extra charged groups. Small settings can move pI enough to change a purification choice during early project screening.